This invention relates to the treatment of articles to provide a wear-resistant, corrosion-resistant surface.
Some steel compositions, termed xe2x80x9cstainless steelsxe2x80x9d, are inherently partially corrosion resistant, which others, termed herein xe2x80x9ccommonxe2x80x9d or xe2x80x9ccarbonxe2x80x9d steels, are not. Articles made of the stainless steels are more expensive than comparable articles made of the common steels. To achieve some of the benefits of stainless steels with reduced cost and also to further enhance some of the stainless steels, the surfaces of steel articles may be treated to improve their corrosion resistance.
In one approach widely used to treat steel articles, such as components of firearms, the article is carefully cleaned in a series of steps, and contacted to a hot, acidic phosphate solution. The phosphate solution reacts with the steel to produce a phosphate-based chemical conversion coating on the surface of the steel. This approach, commonly termed Parkerizing, improves the corrosion resistance of the steel to some corrosive environments. In another approach, the steel article is oxidized in a controlled manner to produce an iron oxide coating, a process termed xe2x80x9cbluingxe2x80x9d after the resulting color. Many other types of corrosion-resistant coatings for steel and other metals are known.
The commonly available coatings for steels and other metals produce improved corrosion resistance, but they are removed relatively easily by abrasion and are therefore less durable than desired. Additionally, many have colors and physical appearances that are unattractive. For example, Parkerizing may result in a relatively rough surface that does not resist wear well.
On the other hand, there are many approaches that increase the wear resistance of surfaces. Common steels are often carburized or nitrided to produce a carbon-rich or nitrogen-rich region at the surface, which is harder and thence more corrosion resistant than the underlying metal. Unfortunately, such wear-resistant coatings do not have good corrosion resistance in many commonly encountered corrosive environments.
There is a need for an improved surface treatment for metals such as steel, which is both wear resistant and corrosion resistant. The present invention fulfills this need, and further provides related advantages.
The present invention provides an article having a surface that is both highly wear-resistant and highly corrosion resistant. The color and appearance of the surface may be varied over a wide range by controlling the processing. If, after a period of service, the corrosion resistance of the article is reduced, it may be restored with a relatively simple, inexpensive treatment. In a preferred embodiment, the surface absorbs and retains oil to further improve its corrosion resistance.
In accordance with the invention, an article comprises a substrate having a surface, and a wear-resistant coating on the surface of the substrate, preferably an intermetallic compound, wherein the wear-resistant coating has micropores therethrough. There is a corrosion-resistant coating on that portion of the surface of the substrate accessible through the micropores.
In the associated processing, a method for preparing an article comprises the steps of furnishing a substrate having a surface, wear coating the surface of the substrate with a wear-resistant coating; and thereafter contacting the wear-coated surface of the substrate to a reactant operable to form a corrosion-resistant inorganic coating on the substrate.
The preferred substrate is a steel, preferably a common steel (as distinct from a stainless steel). The wear-resistant coating on the steel is desirably a titanium nitride-based coating including at least one compound selected from the group consisting of titanium nitride (TiN), di-titanium nitride (Ti2N), titanium aluminum nitride ((TiAl)N), titanium carbonitride (Ti(CN)), titanium aluminum carbonitride ((TiAl)(CN)), chromium nitride (CrN), and zirconium nitride (ZrN), and mixtures thereof. These compounds have good wear resistance and their color and physical appearance may be controllably established by the selection of the compound and its processing. The corrosion-resistant coating is preferably a conversion coating such as a phosphate.
The wear-resistant coatings such as the titanium nitride-based coating impart good wear resistance and, where present on the surface, good corrosion resistance. However, the inventors have observed that there may be, and usually are, small openings or cracks, collectively termed xe2x80x9cmicroporesxe2x80x9d herein, in the wear-resistant coating. The underlying surface of the article substrate is exposed to the external environment through these micropores. The micropores may be present even in carefully coated articles, or the micropores may be intentionally introduced to retain oil therein. Such micropores may result from irregularities in the coating operation or from subsequent cracking of the wear-resistant coating due to its relatively low ductility. Whatever their source, the micropores provide locations for the initiation of corrosion of the underlying surface, leading to its corrosion damage.
Without this knowledge, the formation of the corrosion-resistant coating after the wear-resistant coating has been applied would seem to be redundant, inasmuch as the material of the wear-resistant coating itself has reasonably good corrosion resistance, and there would be no reason to apply the corrosion-resistant coating after the wear-resistant coating. The subsequently applied corrosion-resistant coating has little effect on the surface of the wear-resistant coating, but it has a significant effect in the portion of the surface of the substrate that is exposed through the micropores. The great majority of the surface of the coated article remains the wear-resistant coating, and consequently the article has good wear resistance to both wearing and erosive damage. Yet it also has excellent corrosion resistance because the otherwise-exposed surface of the article is protected.
The present treatment process of application of a wear-resistant coating followed by application of a corrosion-resistant coating is preferably performed on new articles. At a later time, additional micropores may be produced in the wear-resistant coating by hard use or for other reasons. New unprotected surface regions of the substrate are thereby exposed, and corrosion of these regions may result. In that event, the corrosion-resistance coating procedure may be repeated to protect the newly exposed surface regions.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.